It is common practice for a first user on a voice-band telephone network to engage in data communication with a second user on a data network. Usually, these instances involve communication between a first user on the public switched telephone network and a second user on a packet network. The first user's terminal activates a modem which initiates a telephone call and converts the first user's data into voice-band signals. These voice-band signals are transmitted over the telephone network and are in turn converted to data by a modem connected to the packet network. As with most telephone calls, the entire local carrier and long distance costs of the associated voice-band call are generally allocated to the initiator of the call.
Other billing scenarios are possible. For example, U.S. Pat. No. 4,796,292 issued to Thomas discloses several methods whereby the costs of data transmission over a voice-band network can be allocated to the receiving party rather than the initiator of the call. The Thomas patent is specifically directed to the situation where a merchant uses a credit-card reader and model to verify the credit of a card-holder via a data transmission to the credit card company over a voice-hand network.
Problems in call cost allocation arise in an environment where a data connection requires interworking and interconnection between the voice band network and data connections within the ISDN. As an example, a first user on a packet network, such as in Integrated Services Digital Network (or ISDN), has connected his terminal to an ISDN end office via a digital Basic Rate Interface (or BRI) line and needs to establish a data connection with a second user on a voice-band telephone network using a modem. For the data from the first terminal to reach the second user, a call must be first initiated from the packet network user to a device that can make a call on the voice-band network and that can translate the data packets from the packet network to voice-band data signals. Similarly, voice-band data from the second user must be converted to data packets for transfer to the first user.
While the problem described above affects data transfer and interworking using packet capabilities, ISDN also provides a user with a circuit switched digital transmission capability. In a circuit-switched interconnection environment, similar cost allocation problems arise when a call originator on an ISDN establishes a circuit switched data connection to a second data user on the Public Switched Telephone Network (PSTN) using a modem. For the data from the first terminal to reach the second user, a call must be initiated by the first user through the ISDN to a device that can establish the connection over the voice band network and then make the translation from the digital data information to voice band data signals compatible with data modem traffic. Similarly, voice band data from the second user must be converted to digital data compatible with the circuit switched digital connection to the originating caller.
The cost allocation problem arises since the billing identity of the first user may not be readily ascertainable by the voice-band network. The costs of the voice-band call would generally be billed to the device which initiates the voice-band call such as a modem or modem pool, rather than the first or ISDN user. A cost effective method of connecting the first ISDN user to the second voiceband user is necessary which allows for call cost allocation to the true initiating party.
Regional telephone companies have attempted two methods of interconnection involving this packet to voice-band connection scenario. The first method is shown in FIG. 1. In this configuration, the ISDN end office 10 associated with the first user 12 with terminal 14 connected to customer premises equipment terminal adaptor (TA) 16 has implemented a modem pool consisting of ISDN terminal adapters 18 paired with multi-function modems 20. This allows the ISDN user to make a voiceband call via the public switched telephone network (or PSTN) 22 to the Plain Old Telephone Service (or POTS) end office switch 24 and POTS user 20 via terminal 28 and modem 30.
The drawbacks to this first method are that the method is expensive: it requires a special port card in the ISDN switch, a full function terminal adapter and a modem for every port in the modem pool; it requires a modem pool at every ISDN end office that will offer this service; the average usage rate of a modem pool is low; and having a distributed modem pool requires many more idle modems than a centralized modem pool would require.
Further, this method is disadvantageous since it requires the user to dial out in two stages, first to dial the local modem pool, and then to dial the POTS user.
The second method, currently used by several regional telephone companies, is shown in FIG. 2. In this method, the data call from the first user 40 connected to ISDN end office switch 46 via terminal 42 and terminal adaptor 44 is routed through the Public Packet Switch Network (PPSN) 48 to a modem pool that the PPSN normally provides. This modem pool, consisting of individual modems 50, can then create the voice-band call needed to access a second user 60 on the PSTN 52 via POTS end office switch 54, modem 56 and terminal 58.
The drawback to this method is that the modem pool is billed for the voice-band call, not the call initiator, in this case ISDN user 40. The regional telephone companies have tried to work around this problem by only allowing dial out within Local Access and Transport Areas (or LATA). Intra-LATA calls are non-toll calls which would normally not be charged to the ISSN user. This solution severely limits the number of POTS users that can be reached since POTS users outside of the LATA can not be reached.
In addition, regional telephone companies have tried setting up separate modem pools for each ISDN customer. However, this solution requires too many modem pools for individual users and, when used for large businesses, does not allow for the business to bill individual users or departments for the voice-band portion of calls which they initiate.
The problems identified above as a result of using modem pools for interconnection between users on a packet network and users on the voice-band network also apply to instances of interworking between circuit switched digital data users and voice-band data users.